1. Field of the Invention
The present invention relates to a circuit for detecting a pulsive component in a signal. More specifically, the present invention relates to an apparatus for detecting a pulsive component for use in a pulsive noise removing apparatus in an FM receiver.
2. Description of the Prior Art
It has been well known that a pulsive noise such as an ignition noise generated by an automobile could interfere with normal reception by an FM receiver. Since such pulsive noise serves to phase modulate the FM signal, the same cannot be removed even by the use of a limiter and hence is transferred to a subsequent stage in the receiver after detection by a detector. Accordingly, it is necessary to remove such pulsive noise in a signal transmission path subsequent to a detector.
Referring to FIG. 1, there is shown a block diagram of an FM radio receiver employing a typical noise removing apparatus where the present invention can be advantageously employed. Referring to FIG. 1, the FM radio receiver shown comprises an antenna 11 for receiving a broadcast FM signal wave, a radio frequency amplifier 12 for amplifying the FM signal received by the antenna 11, a local oscillator 14 for generating a local oscillation signal for the purpose of frequency conversion, a frequency converter 13 for mixing the amplified FM signal from the radio frequency amplifier 12 with the local oscillation signal for converting the frequency of the FM signal into an intermediate frequency, an intermediate frequency amplifier 15 for amplifying the intermediate frequency signal from the frequency converter 13, an FM detector 16 for demodulating the intermediate frequency signal into the original low frequency signal, a stereo demodulating circuit 17 for demodulating the low frequency signal from the FM detector 16 into the original stereo signal, left and right audio frequency amplifiers 18 and 19 for amplifying the demodulated stereo left and right signals, and left and right loud speakers 20 and 21 for converting the amplified left and right audio frequency signals into the left and right sounds. Detailed structure and operation of the various circuits for the respective blocks are well known to those skilled in the art. Hence, it is not believed necessary to describe the same here in more detail.
In the FM stereo receiver shown, the output of the detector 16 is applied through a noise removing circuit 2 to the stereo demodulating circuit 17. The noise removing circuit 2 basically comprises a delay circuit for delaying, say for 3 to 5 microseconds, the output of the detector 16, a gate circuit 4 for gating the signal to remove a noise component from the delayed output of the delay circuit 3 and a store/pilot signal generating circuit 5 connected to receive the output of the gate circuit 4. The noise removing circuit 2 further comprises a high-pass filter 6, a noise detector 7 and a monostable multivibrator 8 for controlling the gate circuit 4. The high-pass filter 6 is designed to detect the energy of a noise component included in the output of the detector 16 and is adapted to pass the signal component of a frequency higher than the audible frequency. The pulse noise detector 7 is designed to detect a pulsive noise in the output of the high-pass filter 6 and is adapted to trigger the monostable multivibrator 8 upon detection of such pulsive noise. The monostable multivibrator 8 provides an output to the gate circuit 4 for a predetermined time period after the same is triggered. Accordingly, the gate circuit 4 is disabled or opened when the output is obtained from the monostable multivibrator 8, thereby to prevent the signal from the delay circuit 3 from being applied to the stereo demodulating circuit 17 for the above described time period. The store/pilot signal generating circuit 5 comprises a capacitor, not shown, for storing the signal level immediately before the gate circuit 4 is opened and a pilot signal generating circuit, not shown, for generating a pseudo pilot signal for use in stereo demodulation.
A detailed structure of one example of such store/pilot signal generating circuit is seen in U.S. Pat. No. 3,739,285, issued June 12, 1973 to United States Philips Corporation and entitled "CIRCUIT ARRANGEMENT FOR SUPPRESSING INTERFERENCES IN AN FM RADIO RECEIVER." Briefly described, the above referenced U.S. Pat. No. 3,739,285 discloses a store/pilot signal generating circuit comprising a capacitor for storing the signal level at a gate circuit and a parallel resonant circuit connected in series with the storing capacitor. In the following the store/pilot signal generating circuit of the above referenced patent will be described in more detail on the assumption that the same is employed in the FIG. 1 FM receiver. The parallel resonance frequency of the parallel resonance circuit is selected to be the frequency of the pilot signal of the FM stereo broadcasting signal, for example, 19 kHz. Accordingly, the signal level immediately before the gate circuit 4 is opened is maintained in the storing capacitor, while the pilot signal necessary for stereo demodulation is obtained from the parallel resonance circuit as a parallel resonance oscillation signal, which is effective for stereo demodulation in the stereo demodulating circuit 17 in the subsequent stage. With such circuit configuration, the gate circuit 4 is opened when a pulsive noise is received, whereby such noise component is prevented from being applied to the stereo demodulating circuit 17 in the subsequent stage. In addition, when the gate circuit 4 is closed, the signal level maintained by the storing capacitor is obtained, whereby the continuity of the signal is established. Accordingly, the referenced patent is effective in the reduction of a pulsive noise. At the same time, the pilot signal necessary for stereo demodulation is not interrupted and thus stereo demodulation during a time period when the gate circuit 4 is opened is not adversely affected. In spite of the above described advantageous features of the store/pilot signal generating circuit disclosed and claimed in the above referenced U.S. Pat. No. 3,739,285, the same also involves the following shortcomings.
More specifically, with the store/pilot signal generating circuit disclosed and claimed in the above referenced U.S. Pat. No. 3,739,285, a series resonance circuit can also be formed by the storing capacitor and the parallel resonance circuit. Formation of such series resonance circuit, however, causes distortion of the signal being applied to the stereo demodulating circuit 17 at such series resonance frequency. Since the frequency causing the above described distortion, i.e. the frequency of the thus formed series resonance circuit is necessarily lower than the resonance frequency of 19 kHz of the parallel resonance circuit and falls in the audible frequency region, distortion is caused in the sound produced from the speakers 20 and 21. In addition, another problem is caused by virtue of the above described series resonance. More specifically, assuming a case where the signal of a frequency commensurate with the frequency of the above described series resonance circuit is obtained when a pulsive noise is incidentally received, then the gate circuit 4 is naturally opened responsive to the pulsive noise and the signal level at that time is stored in the storing capacitor and thereafter the gate circuit 4 is closed when the signal level as stored is obtained. However, the electric charge that has been charged in the capacitor constituting the parallel resonance circuit is discharged at the same time and as a result a much increased noise component is withdrawn from the store/pilot signal generating circuit 5.
On the other hand, on the occasion of no input signal, the pilot signal obtained from the parallel resonance circuit during a time period when the gate circuit 4 is opened becomes a large level, which is then applied to the stereo demodulating circuit 17. Accordingly, the stereo demodulating circuit 17 is placed in a condition wherein proper demodulation of a left signal or a right signal cannot be performed by virtue of the above described continuous large pilot signal and as a result such phenomenon can be heard as a noise from the speakers 20 and 21.
In order to eliminate the above described shortcomings of the above referenced U.S. Pat. No. 3,739,285, a pulsive noise removing apparatus of a totally different principle was proposed in U.S. Pat. No. 4,066,845, issued Jan. 3, 1978 to the same assignee as the present invention and entitled "PULSIVE NOISE REMOVING APPARATUS FOR AN FM RECEIVER." The second referenced U.S. Pat. No. 4,066,845 is directed to a pulsive noise removing apparatus for an FM receiver comprising a bandpass-amplifier for selectively amplifying a signal of the reference frequency such as the pilot signal freqency of 19 kHz or the subcarrier signal frequency of 38 kHz, and an attenuation circuit for attenuating the output of the bandpass-amplifier at the rate commensurate with the gain of the bandpass-amplifier, without employing a parallel resonant circuit, for the purpose of preventing the pilot signal from being interrupted for a time period when the gate circuit 4 is opened, whereby a positive feedback circuit is formed to the bandpass-amplifier by means of a closed loop including the attenuation circuit and the storing capacitor, so that the bandpass-amplifier cooperates with the positive feedback circuit to serve as an oscillator when the gate circuit 4 is opened, whereby the pilot signal or the subcarrier signal is applied to the stereo demodulating circuit 17 without being interrupted. The U.S. Pat. No. 4,066,845 can achieve the same advantageous features as those achieved by U.S. Pat. No. 3,739,285, while U.S. Pat. No. 4,066,845 totally eliminates the above described serious shortcomings involved in U.S. Pat. No. 3,739,285.
Thus, it has been a conventional practice that a pulsive noise is detected and an input signal is interrupted in being applied to a stereo demodulating circuit for a time period of the pulsive noise, whereby a pulsive noise is removed. The present invention is directed to a pulsive component detecting apparatus that can be advantageously employed in the above described conventional pulsive noise removing apparatus. However, the present invention could provide a variety of applications.
In view of the fact that in an FM receiver usually the white noise becomes relatively larger when a signal of a medium or weak intensity electric field is received, a conventional pulsive noise detecting apparatus usually employed in an FM receiver involved a shortcoming that such a relatively larger white noise on the occasion of reception of a signal of medium or weak intensity electric field is erroneously detected as a pulsive noise. It has been observed that such shortcoming becomes more apparent when a quadrature detector suited for implementation in an integrated circuit is employed as has the detector 7. However, the same applies more or less to a well known ratio detector being employed as the detector 7. In order to prevent such malfunction by virtue of a relatively large white noise, one might think of a decrease of the gain of the amplifier included in the pulsive noise detecting apparatus in association with an increase of the white noise level. However, such approach of decreasing the gain of the amplifier entails another shortcoming in that the dynamic range becomes narrow.